We have seen in recent years growing screen size and application of the liquid crystal display device, and accompanying strong demand for improved visibility (bright, easy to see, high contrast, wide viewing angle, etc.). However, The demand for improved visibility is not sufficiently met by structural improvements of the liquid crystal cell alone. Whether or not the demand can be met depends largely on improvement in performance of retardation films and other optical films.
Hence, the retardation and other optical films are expected to offer high transparency, low photoelastic coefficient, heat resistance, light resistance, high surface hardness, high mechanical strength, large retardation, low wavelength dependency of retardation, and low incident angle dependency of retardation, among other properties.
A conventional method for imparting optical anisotropy to a transparent resin material is film stretching/orientation. It is known that after stretching/orientation, a film of polymethyl methacrylate (PMMA) or polystyrene (PS) exhibits negative birefringence, and a film of polycarbonate (PC) or cycloolefin resin (COP) exhibits positive birefringence. Positive birefringence means refractive index anisotropy in which the refractive index increases in the same direction as polymer molecular chains, part of the film structure, are stretched to orient molecules. Meanwhile, negative birefringence means refractive index anisotropy in which the refractive index decreases in the same direction, and increases perpendicular to that direction, as polymer molecular chains, part of the film structure, are stretched to orient molecules.
Current major resins for retardation films are polycarbonate (PC) (see Patent Documents 1 and 2) and cycloolefin resin (COP), for example, norbornene amorphous polyolefin (see Patent Document 3), all capable of creating large magnitude of retardation.
However, PC retardation films have a high photoelastic coefficient; retardation (retardation value) varies greatly even under small stress. The film cannot be placed under high tension, for example, when attached to other films. In addition, if the film, after being attached, is exposed to high temperature, the heat could induce stress which would in turn cause variation and non-uniformity in retardation. Another problem with the PC retardation film is poor weather resistance.
COP retardation films shows high heat resistance, but problematically poor adhesion.
In contrast, acrylic resin (acrylic polymer), of which PMMA is a typical example, is known to have excellent optical properties. The resin/polymer has been used in various applications as an optical material which provides high light transmittance, low birefringence, and low retardation. However, the acrylic resin inherently produces low retardation; a requisite retardation is hard to achieve by stretching. Furthermore, as liquid crystal displays are more often used in a harsh operating environment than before, there is a growing demand for optical films with high heat resistance. It is nevertheless difficult to give sufficient heat resistance to PMMA stretched film.
The acrylic resin, when made into a film, likely to develop cracks and other defects. A lot of improvements should be made before obtaining adequate mechanical strength, especially, flexibility.
There is on-going research activity to improve heat resistance by introducing various ring structures to the acrylic resin. Resins with improved heat resistance will likely be brittle, making resultant films less flexible.
Film stretching is known to improve flexibility of acrylic resin. By stretching a film, the molecular chains of the polymer making up the film are oriented, and the film comes to exhibit improved flexibility when folded at right angles to the stretching direction.
A retardation film of polymer makes use of the birefringence caused by molecular orientation by stretching. The film is usually manufactured by uniaxial stretching. Uniaxially stretching acrylic resin, however, makes the film less flexible than it should be when folded along an axis parallel to the stretching direction. Biaxial stretching gives sufficient flexibility along all axes, but causes molecules to lose its orientation in in-plane directions. That raises a problem that low birefringence acrylic resin, if biaxially stretched, cannot develop a sufficient in-plane retardation.    Patent Document 1: Japanese Unexamined Patent Publication No 63-189804/1988 (Tokukaisho 63-189804; published Aug. 5, 1988)    Patent Document 2: Japanese Unexamined Patent Publication No. 4-84107/1992 (Tokukaihei 4-84107; published Mar. 17, 1992)    Patent Document 3: Japanese Unexamined Patent Publication No. 6-59121/1994 (Tokukaihei 6-59121; published Mar. 4, 1994)